An Archean quartz arenite – andesite association in the
eastern Baltic Shield, Russia: implications for assemblage
types and shield history
P.C. Thurston
a,*, V.N. Kozhevnikov
baPrecambrian Geoscience Section,Ontario Geological Sur
6ey,933Ramsey Lake Rd.,Sudbury,Ont.,Canada P3E6B5 bInstitute of Geology,Karelian Research Centre,Pushkinskaya,11,Petroza
6odsk,185610,Russia
Abstract
Shallow water sedimentary units are generally considered scarce in Archean greenstone belts. We describe an unusual quartz arenite-subaerial andesite association within the Archean Hisovaara greenstone belt, a fragment of the Parandovo-Tikshozero belt within the Karelian craton of the Baltic Shield. The Hisovaara greenstone belt consists of
several lithotectonic assemblages: (1) a komatiite-tholeiite assemblage\2803 Ma (based on ages of cross-cutting
dikes); (2) an andesite-quartz arenite assemblage cut by similar dikes; (3) an assemblage of coarse volcaniclastic rocks, and (4) an upper mafic assemblage of tholeiitic basalts with minor pyroxene komatiite volcanic rocks. The andesite-quartz arenite assemblage (100 to ca. 750 m thick) has basal amygdaloidal fragmental andesites overlain by massive andesites, then amygdaloidal and plagioclase phyric andesites. Unconformably overlying the andesite is a unit of quartz-rich sandstones (6 – 40 m thick) dominated by quartz arenite extending several km along strike. At the north end, the quartz arenite succession consists of basal andesite overlain by quartz arenite exhibiting hummocky cross-stratification followed by aluminous coarse metasediments and sulfidic argillite and an unconformably overlying
tholeiite/komatiite unit. At the south end, the succession is basal andesite, regolith, cross-bedded quartz arenite,
weathered andesites, a second quartz arenite, argillite and then subaerial rhyolite. REE and HFSE geochemistry has been obtained on the rocks of the andesite-quartz arenite assemblage. The quartz arenites contain low abundances and chondrite normalized patterns vary from relatively fractionated to flat with most of the variation related to grain size, with pebbly units having higher abundance and more fractionated patterns. Combined major and trace element geochemistry indicates that a sodic felsic source with some admixture of mafic material will explain the geochemistry of the quartz arenites. The andesites display moderately fractionated spidergrams with negative anomalies at Ti, Ta and Nb typical of arc volcanism. The andesite-quartz arenite assemblage represents accumulation of shallow water quartz rich sediments in a setting typical of the later stages of arc volcanism in which the volcanic edifice is subaerial at the southern end of the assemblage. However, at the north end, our evidence is interpreted as indicating subaerial andesitic volcanism, subsidence to a shallow marine basin which then deepens and rifts. Therefore the Hisovaara andesite-quartz arenite assemblage provides a linkage in Archean greenstones between assemblages representing
continental volcanism and a platform-to-rift setting. The presence of an erosional interval in\2.8 Ga greenstones
suggests possible pre-2.7 Ga orogeny in the Baltic shield. The pre-2.7 Ga quartzrich sedimentation is similar in age
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* Corresponding author.
to platformal assemblages in the pre-2.7 Ga North Caribou terrane of the Superior Province, Canada. Crown copyright © 2000 Published by Elsevier Science B.V. All rights reserved.
Keywords:Archean; Sediments; Volcanism; Quartz arenite; Andesite; Baltic; Superior
1. Introduction
Quartz-rich metasedimentary rocks and spa-tially associated sedimentary carbonates, typical of stable platforms, are generally scarce in Archean greenstone belts (Ojakangas, 1985). Re-cent work in the Superior Province has revealed
Archean greenstone assemblages containing
quartz-rich sedimentary units in at least three geodynamic settings: stable shallow water plat-forms (Wood et al., 1986; De Kemp, 1987; Thurston and Chivers, 1990), submarine fans with evidence for cannibalization of platformal rocks (Cortis, 1991), and quartz-rich conglomerates and
arenites in pull-apart basins (Born, 1995).
Kozhevnikov (1992) identified a spatial associa-tion of andesites and quartz arenites within
Archean greenstones of the
Parandovo-Tik-shozero greenstone belt near the Karelian Belo-morian collision zone boundary (Glebovitsky,
1973; Volodichev, 1990; Glebovitsky, 1993;
Lobach-Zhuchenko et al., 1995; Glebovitsky et al., 1996) which caused us to investigate two quartz-rich sedimentary assemblages in this region to assess their similarity with possible Superior Province analogues. The andesite-quartz arenite association has not been seen in the Superior Province quartz-rich sedimentary units (Thurston, 1990).
In the classification of assemblages for Archean greenstone belts, quartz-rich sedimentary units (including quartz arenites) are typical of platform
assemblages (Thurston and Chivers, 1990;
Thurston, 1994). The high mineralogical and tex-tural maturity of these rocks, the presence of trough and hummocky bedding in them, associa-tion with stromatolitic carbonates, and, finally,
their occurrence over thousands of km2 in the
Sachigo and central Wabigoon subprovinces, provide a basis for regarding many as members of platform successions formed under shallow-water conditions along a passive continental margin (Thurston and Chivers, 1990).
The literature on Archean greenstone belts shows that similar quartz arenites have been re-ported from platform assemblages in the many cratons (Thurston and Chivers, 1990). They are known in the Dharwar craton, India (Srinivasan and Ojakangas, 1986), in the Bulawayo green-stone belt on the Zimbabwian Shield (Bickle et al., 1975), in the Moodies Group in the Barberton belt of the Kaapvaal craton (Eriksson, 1980), within the Tanzanian craton in the Dodoman system (Kimambo, 1984), in the West African craton (Rollinson, 1978) and in the Yilgarn (Gee, 1982) and Murchison (Watkins and Hickman, 1988) blocks, Australia.
In the Baltic Shield, quartz arenite-bearing as-semblages have been described in some greenstone belts (Fig. 1). In the Koitelainen area, Central Lapland, the sequence which consists of quartz arenites, mica schists, phyllites, volcanic conglom-erates and mafic to ultramafic volcanics has an age of less than 2.7 Ga and rests on ca. 3.1 Ga granitoids (Kroner et al., 1981). It represents a
Lapponian sequence of presumably Lower
felsic volcanics in the course of ore formation. In the Kuhmo belt (Fig. 1), quartz arenites with 2.8 – 3.0 Ga detrital zircons (Hyppo¨nen, 1983) were described from the Hietapera-Kivivaara area (Piirainnen, 1988), where they form part of a felsic volcanic-sedimentary unit in the Juurikkaniemi
Formation which consists of metarhyolites,
metadacites, volcanic breccia, lapilli tuffs, tuffites and tuffaceous turbidites. The age of this unit is
estimated at 2798915 Ma (Hyppo¨nen, 1983).
Northwards, in the Moisiovaara area (Fig. 1), immature sericitic quartz arenites associated with polymictic conglomerates lie between tholeiites and komatiites. An old tonalite – trondhjemite – granodiorite complex and felsic rocks from the
Kuhmo belt are regarded as sources of 2996960
and 28039238 Ma detrital zircons (Hyppo¨nen,
1983) as well as clasts in congomerates (Lu-ukkonen, 1988). In the Hisovaara greenstone belt (Fig. 2), quartz arenites are described in associa-tion with underlying andesites as well as overlying
felsic volcanics and sedimentary rocks
(Kozhevnikov, 1992; Kozhevnikov et al., 1992).
According to Thurston (1990), so far no andesites have been found in platform assemblages. When distinguishing the types of assemblages most re-cently proposed for Archean greenstone belts, the andesite-quartz arenite association revealed in the Hisovaara greenstone belt is considered a fairly rare type of assemblage with a ‘continental’ style of volcanism whose depositional environment is ‘open to speculation’ (Thurston, 1994). However, knowledge of such an assemblage type may be useful in discussing models for the tectonic evolu-tion of greenstone belts and in comparative analy-sis and correlation of Archean cratons.
2. Geological setting
2.1. Regional setting
The Hisovaara greenstone structure is a
frag-ment of the Archean Parandovo-Tikshozero
greenstone belt which extends for 300 km along the Belomoride-Karelide boundary, i.e. along the
boundary between the Karelian granite-green-stone province and the Belomorian collision zone (Glebovitsky, 1973; Volodichev, 1990; Glebovit-sky, 1993; Slabunov, 1993; Lobach-Zhuchenko et al., 1995; Glebovitsky et al., 1996) (Fig. 1).
2.2. Structural geology and metamorphism
The Hisovaara greenstone structure is a syn-form thrown into composite folds, composed largely of supracrustal rocks and surrounded by crosscutting granitoids (Fig. 2). It displays evi-dence for multiple folding events (Systra and
Sko-rnyakova, 1986; Shchiptsov et al., 1988)
subsequently generalized into three deformation stages (Kozhevnikov, 1992). The rocks have suf-fered polymetamorphism of Archean and Sve-cofennian (1.8 – 2.0 Ga) ages, the latter taking place in a high pressure regime with the following
parameters: T=580 – 640°C, P=6.5 – 7.5 kbar
(Glebovitsky and Bushmin, 1983). Archean hy-drothermal processes associated with felsic mag-matism (Kozhevnikov, 1992, 1995) are apparent together with Svecofennian metasomatic rocks
represented by retrograde metamorphism (T=
300 – 350°C) (Bushmin, 1978; Glebovitsky and Bushmin, 1983). The complicated tectonic and metamorphic history of the Hisovaara greenstone belt is largely due to its proximity to the 2.70 – 2.68 Ga Belomorian collision zone (Lobach-Zhuchenko et al., 1995) and later Proterozoic events (1.95 – 1.75 Ga) (Bibikova, 1995).
2.3. Lithologic assemblages
Several major assemblages of supracrustal rocks are distinguished in the Hisovaara green-stone belt (Kozhevnikov, 1992). The northern and southern flanks of the synform differ substantially in character, largely as a function of lateral facies variation, and the types of crosscutting intrusive rocks etc. (Table 1). Because the andesite-quartz arenite association is revealed only on the north-ern flank of the structure, only units on the north flank are described below in detail. The unit ter-minology of Kozhevnikov (1992) is retained in subsequent sections of this paper for ease of ac-cess to the Russian literature.
2.3.1. Lower mafic assemblage
The lower essentially volcanic assemblage is comprised from the base upwards of cumulate peridotitic komatiites, tholeiitic massive and rarer pillow basalts, basaltic to pyroxenitic komatiites and ferrobasalts. This sequence is characterized by thick (up to 10 m) massive and fairly uniform flows with scarce thin-bedded tuff horizons, the absence of interflow sediments, and amygdaloidal textures all indicative of a mafic plateau type of volcanic setting (Thurston, 1994). The U-Pb zir-con age of felsic dykes cutting this assemblage was
estimated by O.A. Levchenkov to be 2803935
Ma (Kozhevnikov, 1992).
2.3.2. Second6olcanic-sedimentary assemblage
The second assemblage consists of volcanics, volcano-sedimentary, and chemical sedimentary rocks of intermediate to felsic composition. Its lowermost unit is comprised of calcalkaline andes-ites. They are overlain by quartz arenites, and at point B (Fig. 2) there is an alternation of andesite and quartz arenite. Resting on the quartz arenites is a thick sequence of felsic rocks including lavas, ash-flows, tuffaceous turbidites and chemical sedi-ments with a clastic component that show compli-cated lateral relationships. Intense metasomatic and deformational processes strongly distort and sometimes obliterate the primary textures and compositions of these strata making interpreta-tion difficult. Where these processes are least in-tense, there are some indications of graded ash flows and pyroclastic breccias as well as flows with massive and flow top breccia textures. The volcano-sedimentary rocks have some features in-dicative of graded rhyolitic turbidites with
alu-mina-enriched upper parts. Transitional
clastic-chemical sedimentary rocks represented by carbonaceous schists (sulfidic argillites), alumino-silicates and cherty rocks occur as thin horizons
and lenses among felsic volcanosedimentary
rocks. The uppermost 100 m of this sequence consists of thin, graded carbonaceous and carbon-ate-bearing silty sandstones.
2.3.3. Third rudaceous assemblage
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Table 1
Some characteristics of associations at the northern and southern flanks of the Hisovaara greenstone belt that illustrate its asymmetric geological structure
Assemblages Northern flank Southern flank
Intensely foliated tholeiitic basalts and minor tuffs. Thickness50.15 km. Komatiitic, tholeiitic and ferrotholeiitic lavas and scarce
Lower Mafic assemblage
tuffs. Thickness 0.5–1.7 km.
Andesitic lavas and pyroclastic flows with indications of
Second Predominantly bedded andesite tuffs. Thickness 0–300 m. Quartz-rich
volcanic–sedimentary subaerial–subaqueous volcanism. Thickness 100–700 m. sediments not found. Felsic volcanics not characteristic. Coarse-bedded assemblage Quartz-rich arenite horizons with hummocky and trough tuffs with oxide- and silicate-facies BIF horizons. Carbonaceous schists cross-bedding. Thickness+n–40 m. Rhyolitic lava and not found. Individual beds and units laterally persistent in thickness, graded bedded ashflows, graded rhyolitic turbidites. bedding parallel in plan view, no indications of cross-bedding and the Carbon-bearing sulfidic argillites, conglomerates, absence of sharp lateral transitions are characteristic.
alumino-silicate and siliceous rocks closely associated with felsic volcanics. Thickness+0–100 m. Graded thin-laminated carbon- and carbonate-bearing siltstones. Variable thickness of individual beds and units as well as complex lateral transitions are characteristic of the association. Indications of cross-bedding are common in the quartz arenites.
Third Rudaceous Thick oligomictic conglomerate (?) or volcaniclastic rock Polymictic conglomerates with dominantly felsic volcanic pebbles of the belt.
assemblage units with tuffmatrix (?) in the east closely associated with felsic lava breccia.
Massive and pillowed tholeiitic basalts with thin
Upper Mafic assemblage Massive and pillowed tholeiitic basalts.
komatiite horizons (?) at the base.
Locally microclinized tonalites rimming the structure in
Intrusive rocks Plagiomicrocline and garnet-muscovitic microcline granites rimming the structure in the south. Scarce rhyolite dykes. Andesite-basalt–dacite sills the north. Rhyodacite-rhyolite dykes and stocks.
not found. Gabbro, gabbro-pyroxenite, granodiorite and komatiite sills Andesite-basalt–andesite-dacite sills. Gabbro,
Fig. 3. Fragmental andesite with 15 – 30 cm fragments of massive andesite with fragments of uniform mineralogic com-position but varying in colour from grey to green. North shore of Lake Verkhnee; hammer 35 cm long.
tion pattern with the sills and dikes concentrated on the northeast side of the belt (Fig. 2, Table 1). As the goal of the present paper is to character-ize and interpret a quartz arenite – andesite associ-ation, uncommon for Archean greenstone belts, these units are described in more detail below.
3. Quartz arenite-bearing assemblage
This assemblage is comprised of three members: calc-alkaline andesites (unit A), quartz arenite horizons (unit Q) and a unit dominated by felsic volcanic, volcano-sedimentary, reworked volcanic and chemical sedimentary rocks (unit F). The boundary between the andesites (unit A) and the underlying Lower Mafic assemblage is most prob-ably a thrust, indicated by intense carbonatization and silicification near the contact as well as marked (up to 1300%) stretching of the andesite (amygdale elongation along the a-lineation paral-lel to dip) (Kozhevnikov et al., 1992). The upper boundary of the assemblage is the unconformity at the base of the coarse clastic assemblage and the upper mafic assemblage.
3.1. Andesite unit (unit A)
3.1.1. Field description
Massive amygdaloidal, glomeroporphyritic and coarse pyroclastic andesites are distinguished. The andesite unit varies markedly in thickness (100 – 700 m) laterally (Fig. 2). In the thickest part of the andesite unit, the following succession of rock types is observed from the base upward: (1) ca. 200 m-thick amygdaloidal, partly coarse, frag-mental andesites (Fig. 3); (2) massive andesites locally showing some vague indications of internal heterogeneity (thickness about 450 m); (3) amyg-daloidal types, seldom with indications of primi-tive pillow textures; thickness on the scale of a few metres to tens of metres; and (4) thick andesite flows with concentrations of plagioclase phe-nocrysts (occasionally glomerophephe-nocrysts)
to-ward the top. The thin glomeroporphyritic
andesite horizon is overlain by the quartz arenite unit that can be mapped laterally for several km. However, there are substantial structural differ-belt as well as thin lenticular units among the
rocks of the previous assemblage. Some indica-tions of discordance between these rocks and the underlying and overlying assemblages are appar-ent in the map patterns (Fig. 2). The dacite-rhyo-lite to rhyodacite-rhyo-lite clasts within these rocks are compositionally uniform and are more felsic than the dacitic matrix. This unit may represent either oligomictic conglomerates or matrix-supported volcaniclastic rocks. The latter interpretation is favoured by their close association with felsic flow top breccia observed at some localities.
2.3.4. Upper mafic assemblage
The fourth essentially volcanogenic assemblage is represented by a thick pile of pillowed tholeiitic basalts with some thin pyroxenitic komatiite flows and sills in the lower part. This unit rests uncon-formably on the second and third assemblages and is cut by individual undated rhyodacite and granodiorite dykes.
2.3.5. Intrusi6e rocks
Near the margins of the belt, supracrustal rocks are cut by tonalites in the north and by granites in the south. The interior of the belt is cut by andesite-dacite sills, rhyodacite-rhyolite (quartz
and quartz-plagioclase porphyry) dykes and
distribu-ences between detailed sections A and B (Fig. 2). The basic difference lies in the fact that on the southern shore of Lake Verkhneye (section B) two stratigraphic subunits of andesites (A1 and A2) are distinguished (Fig. 4). Here, as in section A, glomeroporphyritic andesites are overlain by a cross-bedded quartz arenite unit. Resting on the quartz arenites are two texturally similar andesite horizons, each 6 – 7 m thick. The horizons consist of flow material succeeded upwards by subunits which display vertical grading from pyroclastic breccia to tuff and lapilli tuff. The third horizon, a massive flow is overlain by a ca. 7.0 m-thick quartz arenite bed (Fig. 5). Except for this local-ity, no sedimentary rocks have been found in the andesite unit. All the contacts between andesites and quartz arenites are well-defined and occasion-ally tectonized.
In section B, A.B. Samsonov exposed a contact between glomeroporphyritic andesites and a lower
quartz arenite horizon (Fig. 6). In the ca. 50 cm-thick zone near the contact, weathering in the andesite is indicated by rock disintegration and fine regolith-filled cracks. The andesite immedi-ately beneath the quartz arenite contact displays coarse garnet and quartz stringers not found else-where in the unit. These features suggest modifica-tion of original chemical composimodifica-tion near the contact. At two points, the reddish colour of andesite, observed near the contact within a ca. 1.0 m-wide band, is due to groundwater circula-tion in the more porous contact zone.
3.1.2. Petrography
All varieties of andesites have consistent min-eral compositions. Their distinct crystallization, schistosity, mineral or aggregate lineation and nematogranoblastic structure indicate the meta-morphic nature of the mineral assemblages. The
assemblage green hornblende+plagioclase (An=
Fig. 5. Stratigraphic columns for andesite-quartz arenite asso-ciation in sections A and B. Sample numbers appear to the left of the columns.
23%)+quartz+brown biotite+chlorite+
epi-dote is most common. Magnetite and apatite occur as accessories. Hornblende is clearly oriented along the earlier lineation which is parallel to the dip. The homoaxial replacement of hornblende by colour-less twinned cummingtonite, observed along ca. 10 m-thick subconcordant zones within the andesite unit, suggests a local rise in temperature in these zones. The formation of carbonate (ankerite) near the lower contact with ferrobasalts is related to the circulation of carbon dioxide solutions in the tec-tonically active contact zone. In a ca. 1.0 m-thick zone near the upper contact with the quartz arenite unit, the garnet poikiloblasts with numerous unde-formed rounded quartz inclusions are flattened in the schistosity plane. Also in this zone, large hornblende poikiloblasts are oriented across earlier lineation along the late subhorizontal lineation
which is parallel to theAc-axes of late shear zones.
Fine (up to 2.0 mm) granoblastic quartz veinlets and chlorite rosettes, concordant with schistosity, are observed here. In amygdaloidal andesites, amygdales are filled with milky quartz as well as quartz-plagioclase or quartz – chlorite – carbonate aggregates. Plagioclase plates, up to 1.0 cm in length, that occasionally form stellate aggregates are characteristic of glomeroporphyritic andesites.
3.1.3. Geochemistry
The Hisovaara andesites belong to the tholeiitic magma clan using the scheme of Jensen (1976). With respect to major element geochemistry, the Hisovaara andesites are tholeiitic andesites similar to those of the Blake River Group in the Abitibi greenstone belt (Xie, 1996). Based on the field appearance of the unit in general and the low LOI values, we expected to see little evidence for alter-ation. Two pairs of andesite samples (94-PCT-002 & 004 and 94-PCT-022 & 023) were taken, with one of each pair from within a few cm of the quartz arenite and the other member of the pair from 1 – 2 m beneath the contact. When major element data for the andesites are compared, we observe with increasing proximity to the
con-tact: addition of Fe+3, K, and P, loss of Mg and
Na and variable behaviour of Si, Fe+2, Al, Na and
Ca. Spidergrams of trace element geochemistry (Fig. 7) display a fractionated pattern with
tive anomalies for TiO2, Ta and Nb typical of
island arc volcanism. The negative Hf anomalies in some samples are due to incomplete sample dissolution verified by comparing Zr values ob-tained by XRF with those obob-tained by ICP-MS and corroborated by complete dissolution of simi-lar samples using the closed beaker technique (Jenner, 1996).
REE data for these sample pairs are plotted in Fig. 8a and b. The LREE are quite variable for a suite of samples obtained within a stratigraphic section of a mesoscopically similar rock type a few metres in thickness. In basaltic rocks within a coherent unit, LREE variation is conventionally
ascribed to fractionation of clinopyroxene9
pla-gioclase. No conventional igneous fractionation process will yield sample-to-sample variation in Ce anomalies or crossing REE patterns.
Comparison of the samples immediately under-lying the quartz arenite vs. those somewhat re-moved from the contact reveals: marked LREE depletion, a negative Ce anomaly, some depletion of the HREE and loss of Rb and Cs (Figs. 7 and 8a, b). A sample of the regolith (Fig. 8c) shows negative Ce and Eu anomalies and extreme
deple-tion of the HREE. Precambrian weathering of basaltic rocks (Kimberley and Grandstaff, 1986) results in Na depletion, variable behaviour of the heavier alkalies (K,Rb, Cs) and the alkaline earths (Ca, Mg). Fe shows an upward decrease into the paleosol but this pattern can be disturbed by downward percolation of Fe-bearing ground-water. These authors also report depletion of the LREE in weathering of the Kinojevis basalts in the Abitibi subprovince. In general the behaviour of REE in weathering of mafic rocks seems some-what variable (Braun et al., 1990; Marsh, 1991; Price et al., 1991). Leaching experiments indicate that incipient alteration tends to release REE with behaviour controlled by groundwater parameters (flux, Eh, pH) and the secondary minerals pro-duced during alteration (Price et al., 1991). The proximity of samples 004 and 94-PCT-022 to the quartz arenite contact and the presence of mineralogical changes near the contact and the similarity to the above studies of Precambrian and younger basaltic weathering suggest that the chemical changes we observe may be related to weathering of andesite. If the alteration were due to recent weathering, this cannot explain the
eralogical differences in samples proximal to the quartz arenites, reddening of the fresh surface near the quartz arenite or the more intense alter-ation being in samples taken closest to the quartz arenites. Therefore weathering took place prior to quartz arenite deposition.
3.2. Quartz-arenite (unit Q)
3.2.1. Field description
Quartz arenites associated with andesites were traced over several kilometres on the northern flank of the Hisovaara structure and were studied in detail at some localities (Fig. 2). Quartz arenite unit Q is subdivided into subunits Q1 and Q2. Lower subunit Q1, in contact with the andesite unit, was revealed at all the points shown on the map (Fig. 2). Upper subunit Q2 was found only on the northern shore of Lake Verkhneye in section B. Considerable variations in diverse tex-tures and primary structex-tures, apparent both later-ally and verticlater-ally, are characteristic of unit Q. For example, subunit Q1 varies in thickness from 6 – 8 m (points C and D, Fig. 2) to 10 – 12 (points B, E, 913) and even 40 m in section A. Other differences are apparent in comparing sections A and B. In section A, white fine- to medium-grained thin-laminated quartz arenites rest with a sharp direct contact on glomeroporphyritic andes-ites (Fig. 9). Hummocky cross-bedding with char-acteristic low angles between cross-bedded units and erosion surfaces is observed in some out-crops, depressions being filled with micaceous (argillic) material (Fig. 10). Quartz clasts are pre-dominantly sand sized, except sample 94-PCT-012
that includes small (B1.0 cm) quartz pebbles.
The fairly high degree of rounding of fine quartz pebble material reflects the textural maturity of the rocks. The prevalent white colour of the rocks with a very small admixture of stained minerals indicates the high mineralogical maturity of these quartz arenites. One-centimetre-thick feldspar-rich laminae are preserved locally. Primary stratifica-tion is retained, despite a pressure-solustratifica-tion cleav-age which cross-cuts bedding at a medium angle. The rocks are recrystallized strongly enough to form metamorphic quartz veins, with the degree of recrystallization increasing toward the upper contact of subunit Q1.
Fig. 9. Unconformity between massive andesite (bottom of photo) and overlying quartz arenite. Lighter colour of andesite adjacent to the quartz arenite can be seen. Lens cap, 52 mm in diameter.
tion A. Here, a 10 – 20 cm-thick fine pebble quartz conglomerate horizon lies at its base and has a sharp contact with weathered glomeroporphyritic andesite. White, moderately rounded, flattened quartz pebbles, up to 3.0 cm in size along the long axis, are closely packed and supported by sand-sized mica and quartz matrix. These rocks gradu-ally pass upwards to fine pebble arenites. Poorly rounded, often angular pebbles measuring 1.0 – 1.5 cm are represented solely by white quartz. The grey quartz arenite matrix consists of unequally rounded commonly angular quartz grains, 1.0 – 2.0 mm in size, with the addition of biotite, which suggests the low textural and mineralogical matu-rity of the rock. In this unit, which has an ex-posed thickness of ca. 7.0 m, trough bedding, most distinct in its lower half, is obvious. The upper contact between subunit Q1 and the andes-ites of subunit A2 is not visible because it is covered by Quaternary deposits.
In section B, both subunits are represented, with subunit Q1 markedly differing in some char-acteristics from its stratigraphic analogue in
The second quartz-rich rock horizon (subunit Q2) is up to 7.0 m thick. It rests with a sharp contact on the weathered andesites of subunit A2. Subunit Q2 differs in some macroscopic features from subunit Q1. It is characterized by:
1. The yellowish, locally rusty colour of the rocks caused by the presence of finely dispersed al-tered iron sulphides.
2. Thin, locally deformed parallel lamination. 3. A dominant sand size and smaller dimensions
of quartz grains and scarce thin (10 – 15 cm)
horizons containing fine (B1.0 cm) quartz
pebbles (sample 94-PCT-015).
4. The presence of ca. 30 cm-thick horizons with
very fine-grained (B0.05 mm) quartz (sample
94-PCT-021).
5. The occurrence of thin (10 – 15 cm) muscovite-enriched horizons (sample 94-PCT-019) re-sponsible for the graded nature of individual beds of this subunit.
Resting directly on subunit Q2 are 2.0 – 3.0 m of carbonaceous rocks overlain by felsic rocks that represent rhyolitic ash flows.
3.2.1.1. Petrography. Highly siliceous rocks are divided based upon microscopy into several types that differ in the nature of clastic material, the degree of quartz recrystallization, and the rela-tionships between quartz and other silicates etc.
Quartz arenites, the most abundant rock type of the unit, are bedded rocks in which quartz-rich beds alternate with beds that contain quartz and other silicates. Ninety to ninety-five per cent of the rock consists of quartz grains varying from 0.2 to 2 – 3 mm in diameter. Intense recrystalliza-tion gives rise to the less common sutured polygo-nal boundaries of grains in quartz monocrystals and deformation that continues until lenticular aggregates are formed. It is, therefore, impossible to estimate the degree of roundness of fine clastic quartz. Two types of plagioclase are observed in the quartz arenites: (a) fine, poorly rounded clas-tic grains are commonly filled with grey dust-like opaque material along cleavage cracks, which is presumably due to weathering; (b) newly-formed plagioclase grains occur together with garnet and amphibole in the form of fine chains that fill interstices between quartz grains. Trace minerals
occur as newly-formed grains or chains of mica, amphibole, garnet, kyanite, staurolite and chlorite aggregates. Zircon, sphene and opaque ore miner-als occur as detrital accessories.
Pebbly quartz arenites are second in abundance in the section. Unlike the quartz arenites de-scribed above, they contain polycrystalline quartz interpreted as vein quartz strongly elongated
along the Acaxis which is parallel to the mineral
lineation and dip of the rocks. In cross section, normal to lineation, various shapes of quartz pebbles (subrounded to angular, but generally
poorly rounded) that vary in size from 0.2×1 –
1×2 cm (over 5 cm along lineation) in these
cross-sections are easily observed in sawed slabs. Individual quartz pebbles constitute thin centime-tre-scale laminae in quartz arenites that provide the matrix for coarser quartz. Trace minerals and accessories are similar to those described above, but their quantities vary markedly. For example, pebbly quartz arenites from section B contain more biotite, and at station 913 zircon is present in large amounts (about 30 grains in one thin section).
In subunit Q2, two more rock types were seen
in section B. In sample 94-PCT-021, fine
equigranular quartz (grain sizeB0.05 mm) in
which individual coarser (1 – 2 mm) deformed de-trital grains are observed. The quartz arenites pass upwards into thin-laminated muscovitic quartz arenites. This subunit includes chemical sediments and tuffaceous material.
neg-Table 2
Mean chemical compositions of quartz-arenites from Hisovaara and other Archean regionsa
1 2 3 4 5 6 7 8
94.5 92.13
SiO2 94.4 95.97 87.02 88.65 92.11 93.45
0.05 0.06 0.02 0.1
0.02 0.01 0.01 0.08
0.02 0
MnO 0.01 0.02
0.42
MgO 0.35 0.31 0.2 2.47 0.44 0.88 0.27
0.26 0.26 0.07 2.98
CaO 0.43 0.1 0.09 0.14
0.46 0.98 0.04 0.07
0.76 0.36
38.8 21.2 42.8 19.5
42.7 11.5
aRecalculated to 100% on a volatile-free basis. (1–4) Quartz arenites from Hisovaara: quartz arenites, subunits Q1 (1) and Q2 (2); pebbly quartz arenites, subunits Q1 (3) and Q2 (4). (5–6) Quartz arenites, Keewaywin formation (5) and Keeyask Lake Formation (6), Sandy Lake greenstone belt Superior Province, Canada (Cortis, 1991). (7) Quartz arenites, Pongola supergroup, S. Africa (Wronkiewicz, Condie, 1989). (8) Quartz arenites, Yavanahalli belt, S. India (Argast and Donnelly, 1982).
ligible amount of plagioclase and the absence of lithic fragments suggest that in a quartz-feldspar-lithic fragment sandstone provenance, the compo-sitions of the quartz-rich metasediments of the quartz arenite-andesite assemblage at Hisovaara lie on the ‘quartz-feldspar line’ near the quartz apex (i.e. in field 1), suggestive of a cratonic provenance (Dickinson, 1985).
3.2.1.2. Geochemistry. The major and rare earth element geochemistry of quartz arenites and asso-ciated rocks from the Hisovaara greenstone belt is shown in Tables 2 – 4. Both field and microscopic characteristics are used to define three groups: quartz arenites Q1 and Q2 and pebbly quartz arenites Q1. Pebbly quartz arenite (sample
94-PCT-015) and mica quartz arenite (sample
94PCT-019) from subunit Q2 were analysed sepa-rately. In the above groups, some major
compo-nents such as SiO2, Al2O3, FeO, Fe2O3, CaO,
Na2O and K2O vary over a wide range. Variations
in TiO2and MgO content are less appreciable. We
compare the average compositions of the Hiso-vaara quartz arenites with similar rocks from other Archean regions (Table 2) but they have some distinctive characteristics. For example,
Hisovaara quartz arenites contain more SiO2,
CaO and Na2O and less Al2O3 and K2O. Their
SiO2/Al2O3ratio is higher and K2O/Na2O ratio is
lower. The high CaO content of the quartz arenites in the Keewaywin Formation is obviously due to superimposed carbonatization (Cortis,
1991). The CIA value (CIA=[Al2O3/(Al2O3+
CaO+Na2O+K2O)]×100; Nesbitt and Young,
1982) estimated for this group is not given be-cause in the calculations CaO represents Ca in a silicate form (McLennan et al., 1990). Many
sam-ples collected in subunit Q1 show ultralow (B0.2)
K2O/Na2O ratios (Fig. 11). The above
character-istics of the rocks described, emphasized by the
strong predominance of Na2O over K2O, indicate
that the matrix of Hisovaara arenites, in which undecomposed plagioclase grains play a major role and the pelite component is less significant and chemically immature. The only exception is mica quartz arenite (sample 94-PCT-019) which is
abnormally poor in SiO2 and abnormally rich in
Al2O3 and K2O.
indi-Fig. 11. SiO2/Al2O3-K2O/Na2O plot for quartz arenites.
cates the lower chemical maturity of quartz arenites at Hisovaara. The lowest mean CIA value of 52 was determined for the quartz arenites of subunit Q1. A fairly unusual combination of low
chemical maturity and very high SiO2 content,
observed in the rocks discussed, is largely respon-sible for their trace element geochemistry.
Analysis of REE content and REE distribution patterns has revealed some distinctive features of sedimentary rocks at Hisovaara. First of all, it
should be noted that theirSREE values are much
lower than those of Pongola quartzose sandstones (Fig. 12). Extremely small quantities of rare earths
(SREE=4.46 – 15.20 ppm) were determined for
the quartz arenites of subunit Q1. The pebbly arenites of both subunits and the quartz arenites of subunit Q2 contain REE in much greater quan-tities. The REE content of mica quartz arenite in
sample 94-PCT-019 is abnormally high (SREE=
96.76 ppm). This supports the conclusion that REE dominantly form part of micas, i.e. the argillic matrix of quartz arenites (Wronkiewicz and Condie, 1989). All rock samples show a
nega-tive Eu anomaly Eu* (Eu*=Eun/(Smn,Gdn)1/2)
value of 0.67 (Taylor, 1979). About 80% of
Archean sedimentary rocks have Eu*]0.85
(McLennan et al., 1984, 1990).
There are marked differences in REE distribu-tion pattern between the groups studied. This primarily applies to the quartz arenites of sub-units Q1 and Q2. In subunit Q1, two types of samples are distinguished. Type 1 (samples 94-PCT-005 and 011) is characterized by slightly
fractionated REE distribution (LaN/YbN=2.78 –
4.32), flat HREE distribution (GdN/YbN=0.9 –
Fig. 13. Chondrite normalized REE profiles for Hisovaara quartz arenites. (A) Unit Q1 type 1; (B) unit Q1, type 2; (C) unit Q2 fractionated REE patterns; and (D) pebbly quartz arenites.
1.02), a higher Eu/Eu* value (0.75) and a
mini-mum SREE value (4.46 – 6.41 ppm) (Fig. 13A).
Type 2 (samples 94-PCT-007, 008 and 010) is
characterized by a higher LaN/YbN ratio (8.82 –
12.15), more fractionated SREE distribution
(GdN/YbN=1.74 – 2.0), lower Eu/Eu* values
(0.59 – 0.74) and higher SREE values (10.23 –
15.20 ppm) (Fig. 13B). Subunit Q2 shows even more diverse REE characteristics. For example, sample 94-PCT-016 is fairly similar in REE
frac-tionation pattern (LaN/YbN=3.91, GdN/YbN=
0.92) to samples of type 1 from subunit Q1. However, its higher REE value is observed to-gether with a far more intense negative Eu
anomaly (Eu/Eu*=0.59) (Fig. 9A). Samples 018
and 020 occupy an intermediate position in terms of REE characteristics between types 1 and 2 in
subunit Q1 (LaN/YbN=8.29 – 8.51, GdN/YbN=
1.20 – 1.49, Eu/Eu*=0.70 – 0.73) (Fig. 13B).
Sam-ples 94-PCT-019 and 021 show maximum REE
(LaN/YbN=24.57 and 25.43) and SREE (GdN/
YbN=3.08 and 2.76) fractionation, but they
dif-fer markedly in Eu/Eu* ratio (0.82 and 0.59,
respectively) (Fig. 13C). In both subunits, pebbly quartz arenites generally have more persistent
REE and SREE fractionation patterns, but their
Eu/Eu* values vary substantially (0.53 – 0.80) (Fig.
13D). Sample 94-PCCT-017 from altered felsic tuff differs greatly in REE distribution from the rocks described. It has a slightly fractionated
(LaN/YbN=2.46) REE distribution, marked
SREE enrichment (GdN/YbN=0.76) and a
pro-nounced positive Eu anomaly (Eu/Eu*=2.00)
(Fig. 13C).
In the GdN/YbN– Eu/Eu* diagram (McLennan
anomaly. Partial overlap with the Archean
sedi-mentary rock field is observed in the low GdN/
YbNrange (Fig. 14A).
Th and U content varies between and within the rock groups differentiated (Fig. 14). In sub-unit Q1, quartz arenites contain less U (average U content is 1.02 ppm) and especially Th (average Th content is 3.62 ppm) than pebbly arenites
(U=1.57 ppm, Th=8.95 ppm). One exception is
sample 94-PCT-013 with an abnormally small amount of U (0.49 ppm) and, correspondingly, an
abnormally high (15.1) Th/U ratio. The quartz
arenites of subunit Q2 are markedly richer in both
elements (U=1.58 ppm and Th=7.28 ppm) than
the quartz arenites of subunit Q1. Th/U values
vary from 2.7 to 15.1, extremely high values being due to either extremely low U content (sample 013) or abnormally high Th content (sample
94-PCT-021). Generally speaking, in Th versus Th/U
coordinates (McLennan and Taylor, 1991) the Hisovaara quartzose sandstones plot completely within the Archean sedimentary rock field. Th and U distribution seems to be largely controlled by the distribution of heavy minerals, primarily zircon, indicated by the fact that anomalous
quantities of some trace elements (Zr=933 ppm;
Th=338 ppm; Y=57 ppm and Pb=72 ppm)
that can only be explained by the accumulation of relatively abundant zircon as found in sample 913 (Kozhevnikov, 1992).
3.2.1.3. Interpretation. The above geological, pet-rographic and geochemical data on the quartz arenite unit can be discussed from two aspects. One aspect is related to the source of both quartz and the other detrital components which consti-tute the unit. The other aspect is the
reconstruc-tion of the depositional environment and
depositional mechanism of these rocks that are the oldest sedimentary rocks in the Hisovaara greenstone belt.
With respect to the provenance of the quartz arenites, identification of class types is the most informative data set. Hisovaara rocks contain no lithic fragments that directly indicate possible sources. Therefore, data on the geochemistry of immobile elements such as REE, major element chemistry, abundance of quartz pebbles and some textural characteristics of the rocks discussed are critical.
Vein quartz, normally present in addition to other types of rock fragments, has been reported from practically all Archean quartz-rich sediments (Eriksson, 1980; Srinivasan and Ojakangas, 1986; Bhattacharyya et al., 1988; Wronkiewicz and Condie, 1989; Cortis, 1991, etc.). Several rock types can be proposed as hypothetical sources.
Geochemical, mineralogical and other data place a limit on some sources that are probable in principle. It should be noted that when using REE geochemistry to constrain source(s) of the clastic material in the quartz arenites, we assume that:
1. Quartz-rich clastic rocks adequately reflect source area geochemistry. This follows from flat REE distribution patterns in quartz sands normalized in terms of the REE content of associated muds in both present-day passive margin environments (Biscay, Ganges) and ac-tive continental margin settings such as back-arc basins (Japan) and continental back-arcs (Java) (McLennan et al., 1990).
2. The lowSREE content of Hisovaara rocks is
presumably due to the diluting effect of quartz. This phenomenon was used to explain
small quantities of SREE in Archean quartz
arenites (McLennan et al., 1984; Wronkiewicz and Condie, 1989) and the commonly ob-served low REE content of modern sands in comparison to that of associated muds, in
which La/Yb, La/Sm and Gd/Yb ratios being
either unchanged or slightly disturbed
(McLennan et al., 1990).
3. The role of vein quartz in the REE
distribu-tion pattern, i.e. LaN/YbN, LaN/SmN, GdN/
YbN and Eu/Eu* values, is presently
impossible to assess properly because there are no data on these parameters for the vein quartz of Hisovaara rocks. The relevant evi-dence, in the literature, is scanty (Siddaiah et al., 1994). Therefore, such an assessment is of limited value.
4. Low CIA, low K2O/Na2O and Al2O3/Na2O
and high SiO2/Al2O3 values observed in the
Hisovaara quartz arenites indicate that the initial geochemical parameters of the felsic source area(s) are retained better here than in similar rocks from other regions.
The chemical-exhalative mechanism for
devel-opment of a high SiO2 concentration in a
hypo-thetical source of clastic quartz cannot really be applied to Hisovaara rocks because of their dis-tinct negative Eu anomaly which is sharply posi-tive in chemically precipitated rocks (Bavinton and Taylor, 1980; Siddaiah et al., 1994). An ad-mixture of chemically precipitated material is
pos-sible, in principle, in sample 94-PCT-017 which has slightly fractionated REE distribution and a strong positive Eu anomaly.
Weathering of quartz-rich amygdaloidal andes-ites could provide a source of quartz. It could accumulate in some settings, e.g. a nearshore beach zone. However, the possible mechanism for such complete andesite decomposition, needed to explain the complete absence of lithic fragments, remains obscure. Furthermore, weakly positive Eu anomalies in andesites do not favour this option.
Quartz-rich metasomatic rocks could be a source of quartz, as observed, for example, in some Proterozoic rocks in Karelia (Kozhevnikov and Golubev, 1995). Fragments of metasomatic rocks, fuchsite schists, tourmaline quartz arenites and other rocks are found in some Archean belts (Luukkonen, 1988; Cortis, 1991; Kozhevnikov, 1992). Metasomatic quartz rocks usually contain no plagioclase, and the rocks consist of a quartz-mica association, Ca and Na being completely removed. The presence of plagioclase in the detri-tal material of quartz arenites at Hisovaara and their low CIA value does not seem to favour a metasomatic source. The REE distribution pat-tern of most of the samples analysed indicates that the major constituent of the source was rep-resented by felsic rocks, which is reflected in LREE enrichment, fractionated HREE distribu-tion and a negative Eu anomaly. Their sodic nature, which is retained even during partial weathering, suggests that they could be tonalite-type granitoids or felsic volcanics of a sodic series. Judging by the presence of at least two types of detrital zircon in the samples analysed, the felsic source is assumed to be complex. Some problems that arise when the ‘quartz budget’ in quartz-rich sediments is estimated using a granitoid destruc-tion mechanism (Pettijohn et al., 1972) can be overcome by assuming that hypabbysal subvol-canic bodies, typically containing abundant vein quartz, were destroyed.
employed as end members. Komatiites were in-cluded in calculations for several reasons. Firstly, it has been found earlier that the Cr content of subunit Q1 varies over a very broad range from 42 to 581 ppm (Kozhevnikov and Travina, 1993). Secondly, thin (maximum 15 cm) Cr-enriched metasandstone horizons that contain finely dis-persed metamorphic fuchsite were observed in quartz arenites at several levels above quartz arenite subunit Q, which testifies to the presence
and weathering of an ultramafic source
(Kozhevnikov, 1992). Thirdly, flat HREE
distribu-tion and slight LREE enrichment at low LaN/YbN
and a strong negative Eu anomaly require the use of an additional component for interpretation. This component must have characteristics most similar to those of the komatiites in the lower mafic association.
Table 5 shows the chondrite-normalized REE content of the rocks that hypothetically form the end members of possible sources (tonalites, komati-ites and rhyolkomati-ites), for a series of quartz-rich samples from subunits Q1 and Q2 and in estimated mixtures diluted with quartz containing negligible REEs. Estimated normalized REE values are sim-ilar to those observed in the Hisovaara quartz arenites. Their distribution curves are similar, too (Fig. 15). This suggests that such REE distribution, e.g. those observed in samples from unit Q1, could be indicated by the deposition of the products of destruction of a bimodal source with various ratios of felsic to ultramafic components. Tonalite de-struction products were deposited dominantly within individual thin horizons (sample 94-PCT-007). In the course of subunit Q2 formation, the role of a felsic source increased steadily, the degree of quartz dilution being possibly lower.
Intense weathering of tonalites and ultramafic rocks must have followed the uplift and deep erosion of the roots of the greenstone belts domi-nated by the rocks of the mafic assemblage, includ-ing komatiites and hypabbyssal tonalitic plutons saturated with vein quartz. In this case, conditions favourable for the concentration of the most resis-tant rock destruction products, e.g. quartz, heavy minerals and partly plagioclase, must have been formed locally. A river channel in a deeply eroded mountain system or in any other small, seasonally
drained basin is a possible environment. It seems to be the type of setting in which clastic (e.g. pebbly) quartz material could be abraded for a short time and the multiple intense scour of deposits, accom-panied by the formation of quartz sands and coarse gravel, could occur. The proximity of source(s) of clastic material and its very rapid transport without long abrasion is indicated by:
1. The textural immaturity of pebble-sized clasts. 2. The occurrence of unrounded, euhedral zircon
fragments.
Some characteristics of the quartz arenite accumu-lation field can be reconstructed by summarizing the above evidence. Judging by the bedding pat-terns in quartz-rich and associated rocks, they were deposited in a marine setting. In section A quartz arenites, hummocky cross bedding could be formed in a shelf zone affected by contour currents and storm waves at a depth up to 90 m, i.e. above the storm wave base (Duke, 1985). Other possible environments for the occurrence of hummocky cross stratification are known [flash-flood braided delta (Hjellbakk, 1993), eolian systems Langford (1989), and antidunes in a fluvial channel (Rust and Gibling (1990))]. The angularity of quartz pebbles is presumably due to limited transport distances. Furthermore, the transition to overlying sulfidic argillites, could represent basin deepening e.g. Hoffman, 1987) or development of lagoonal condi-tions. This transition could be rapid enough for the burial of texturally immature quartz-rich rocks. The trough cross-bedding, observed in the quartz-rich sandstones of section B, could form in small channels near the shoreline (Mueller and Dimroth, 1985). Association of these rocks with subaerial andesites and rhyolites favours such a setting.
The two andesite units (Fig. 5) differ in texture, but are identical geochemically, thus indicating similar magma-generating conditions. This points to a short time interval between the two episodes of andesite volcanism, the succession of processes being: andesite volcanism — a non-depositional interval — the formation of a thin weathered crust-rapid transport and deposition of quartz rich sedimentary rocks. Addition of tuffaceous material to subunit Q2 indicates that sedimentation associ-ated with the completion of andesite volcanism
P
Whole-rock chemical analyses (wt%) from Hisovaara
6 7 8 9 10 11 12 13 14 15 16 17 18
1 2 3 4 5
94-PCT-
94-PCT-92-8 92-12 92-14
94-PCT-92-10 92-16 94-PCT- 094- 92-35 94-PCT- 92-37 92-38 94-PCT- 92-36 94-PCT-
94-PCT-003 012
012
008 003
011 010 007 PCT-05
93.80 94.40 96.08 96.48
SiO2 91.92 92.00 93.00 93.24 03.50 96.98 87.52 88.42 91.80 91.94 92.06 92.64 93.48 94.30
0.08 0.02 0.02 0.02 B0.01 0.01 0.01 0.10 0.08 0.04 0.06 0.07 0.06 0.03 0.04
TiO2 0.06 0.06 0.02
2.35 2.35 1.70 1.84 1.18 6.72 6.15 4.20 4.45
3.28 4.46
2.62 2.31 3.15 2.62 2.75
Al203 2.36 2.03
MnO 0.10 0.02 0.00 0.02 0.02 0.01 0.01
0.35 0.20 0.20 0.15 0.45 0.30 0.60 0.20
MgO 0.75 0.80 0.65 0.40 0.25 B0.05 0.30 0.30 0.60 0.20
0.63 0.42 0.07 0.07 0.07 0.42 0.07 0.52 0.42
0.07 0.07
0.98 0.50 0.07 0.07
CaO 1.12 0.49 0.30
169
1.25 0.85 0.30 1.00 0.85 0.35 0.63 0.56 0.12 0.36 0.03 0.52 1.50 0.87 0.55 0.87 0.06
Na2O
0.05 0.31 0.06 0.10 0.21 2.23 0.19 1.15 0.33
K2O 0.04 0.04 0.62 0.07 0.03 0.91 0.43 0.38 0.90
0.08 0.10 0.05 0.02 0.07 0.20 0.10 0.10 0.15
0.05 0.04
0.08 0.07 0.15 0.95
H2O 0.10 0.08 0.04
0.14
0.25 0.27 0.28 0.20 0.16 0.23 0.17 0.19 0.11 0.76 011 0.58 0.18 0.28 0.56 0.21 0.35
L.O.1
P2O5 n.d.* n.d. n.d. 0.24 B0.01 n.d. n.d. B0.01 B0.01 B0.01 0.05 B0.01 0.03 0.03 B0.01 0.03 B0.01 0.01 100.03 99.95 100.19 100.16 100.03 99.80 99.95 99.77 99.71
99.77 99.89
99.62 99.76 99.84 99.76 99.68
Total 100.08 99.78
53
37 36 56 50 47 58 58 61 68 64 54 58 55 64 58 57 69
CIA
SiO2/ 38.95 45.32 35.50 40.36 28.51 39.91 40.17 56.52 52.43 82.19 13.02 14.38 21.86 20.66 20.64 2941 35.68 34.29 Al2O3
0.02 0.06 0.89 0.10 0.18 1.75 6.19 0.06 2.21
2.07 0.07 0.22 105 0.78 0.44 15.0
0.05 0.03 K2O/
Na2O
1.94 2.76 6.71 2.70 3.28 9.82 18.67
Al2O/ 1.89 2.39 8.73 2.31 2.03 8.08 3.00 5.13 8.73 3.01 45.83
94-PCT- 94-PCT- 94-PCT-
94-PCT-913 94-PCT- 94-PCT- 94-PCT- 578-2 577-2 92.39 94-PCT- 94-PCT- 94-PCT- 576-4 831-1
016 021 019 24 017
018 004 002
020 023 022 014
021
95.44 77.76 78.30 66.00 57.88 57.98 56.72 57.10 56.40 57.00 56.02 60.42 37.75
SiO2 94.36 21.10 93.48 95.20 96.20 67.00
0.02 0.12 0.35 0.72 0.77 0.60 0.84 1.32 1.30
0.02 1.04
TiO2 0.05 0.07 0.08 0.04 1.42 0.64 0.31 0.35
2.23 13.02 12.38 17.74 13.39 13.25 15.73 14.48 14.88 14.20 14.41 14.27
Al2O3 2.09 3.11 2.84 2.07 1.70 6.24 16.40
0.16 0.52 0.88 2.04 1.65 3.56 11.60t 2.99 2.82
0.44 2.92
0.84 0.12 3.44 2.02 1.90 1.10
Fe2O3 1.55 0.68
0.50
0.50 1.01 0.50 1.15 0.57 1.15 1.01 1.94 7.64 6.36 – 8.38 8.98 8.62 9.05 7.54 4.57 2.15
FeO
0.01 0.01 0.02 0.04 0.12 0.17 0.14 0.13 0.11
MnO 0.01 0.02 0.03 0.01 0.01 0.15 0.12 0.11 0.27 0.05
0.20 0.61 0.40 1.00 4.33 4.44 2.11 3.02 3.98
0.25 4.03
0.10 2.62 3.33 26.40 1.43
MgO 0.40 0.36 0.40
0.07
0.07 0.42 0.42 0.14 0.07 0.42 0.70 2.10 6.03 6.20 6.16 5.68 6.10 4.28 6.00 4.51 5.74 3.27
CaO
0.05 0.15 0.04 0.32 1.51 2.42 5.17 5.98 5.47 4.33 3.45 5.15 4.01 5.17 0.08 5.88
Na2O 0.07 1.22 0.41
0.65 4.00 2.75 2.91 0.08 0.26 0.27 0.52 0.33
0.42 0.40
0.76 0.51 0.23 0.02 1.22
K2O 0.29 0.38 0.59
0.05
0.07 0.07 0.03 0.02 0.11 0.03 0.10 0.23 0.09 0.66 0.15 0.15 0.33 0.19 0.22 0.06 0.29 0.14
H2O
0.28
0.59 0.40 0.50 0.10 0.20 1.79 1.39 2.23 2.56 0.71 0.35 1.18 1.47 1.49 1.45 1.37 11.51 0.83
L.O.1
B0.01 0.01 0.06 0.16 0.17 0.22 0.24 0.24 0.22
B0.01 0.18
P2O5 0.01 0.20 n.d. B0.01 0.26 0.17 n.d. 0.15
99.70 99.77 99.85 99.52 99.94 99.74 99.78 99.52 100.37 99.65 99.53 99.87 100.08 99.96 Total 100.23 99.98 99.87 99.89 100.09
71 70
73
50 61 68
CIA 66
46.00
45.15 29.61 32.92 56.59 42.80 5.97 6.32 3.72
SiO2/ Al2O3
2.80 16.25
K2O/ 10.86 0.24 0.93 11.80 12.5 1.82 1.20
Na2O
11.33 55.75 40.69 8.20 7.33 6.93 41.4
Al2O3/ 29.86 2.55 Na2O
an.d. Not detected. t-Fe recalculated to Fe
P
94-PCT-011 94-PCT-010 94-PCT-008 94-PCT-007 94-PCT-005 94-PCT-012 94-PCT-013 94-PCT-003 94-PCT-001 94-PCT-021 94-PCT-020
7.53 5.19 4.31 8.17 7.16 6.14
1.96
La 1.12 3.06 2.23 0.70
2.59 6.59 5.04 1.67 15.48 10.46 9.50 13.95 13.52 11.21
Ce 4.35
0.52 0.22 1.84 1.19 1.06 1.93 1.72 128
0.78
Pr 0.33 0.57
6.67 4.16 3.73 6.74 6.08 4.72
0.90
1.95 1.96
Nd 1.25 2.94
0.22
0.27 0.58 0.35 0.40 1.17 0.67 0.68 1.12 0.93 0.85
Sm
0.21 0.11 0.16 0.16 0.15 0.19
Eu 0.06 0.12 0.06 0.08 0.05
0.91 0.52 0.55 0.89 0.65 0.74
0.19 0.22
Gd 0.42 0.28 0.33
0.02
0.03 0.05 0.03 0.04 0.13 0.06 0.07 0.11 0.08 0.11
Tb
0.61 0.26 0.34 0.56 0.33
Dy 0.18 0.24 0.17 0.22 0.13 0.58
0.12 0.05 0.06 0.11 0.06 0.11
0.03 0.04
Ho 0.03 0.05 0.03
0.33 0.14 0.18 0.32 0.17 0.37
Er 0.11 0.14 0.10 0.13 0.10
0.04 0.02 0.02 0.04 0.02 0.06
0.02
Tm 0.02 0.02 0.01 0.02
0.17
0.17 0.17 013 0.15 0.34 0.18 0.25 0.39 0.19 0.50
Yb
0.06 0.04 0.04 0.06 0.03 0.08
0.04 0.03
Lu 0.03 0.02 0.03
0.46
6.41 15.20 10.87 10.23 35.44 23.05 20.95 34.55 31.09 26.94
SREE
2.78
4.32 12.15 11.58 8.82 14.95 19.47 11.64 14.11 25.43 8.29
LaN/YbN
4.05 4.88 4.00 4.60 4.85 4.55
2.00
3.32 4.01 3.09
LaN/SmN 2.61
1.78
1.02 2.00 1.74 0.90 2.16 2.34 1.78 1.84 2.76 1.20
GdN/ YbN
0.62 0.57 0.80 0.49 0.59 0.73
Eu/Eu* 0.75 0.74 0.59 0.67 0.75
9.33 7.39 8.46 10.62 14.59 6.35
3.32 4.38
Th 5.18 21.38 2.84
1.12
1.24 1.29 0.72 0.71 1.78 0.49 1.78 2.22 1.95 1.64
U
94-PCT-017 568-2 577-2 94-PCT-004 94-PCT-002 94-PCT-023 94-PCT-022
94-PCT-018 94-PCT-016 94-PCT-015 94-PCT-019 94-PCT-024 94-PCT-014
6.89 24.64 4.09 4.24 9.73 14.63
28.83 14.45
La 3.28 3.30 5.79 21.11 3.56
690 716 12.66 58.82 145 46 50.80 12.42 9.81 23.35 49.79 39.43 9.50
Ce 43.54
1.93 6.14 1.71 1.78 3.00 4.59
6.70 4.40
0.82 1.58
Pr 0.84 1.46 5.04
23.63
2.96 3.14 5.26 18.02 706 25.70 857 8.71 12.70 19.68 18.49 8.07
Nd
0.57 0.70 0.89 3.45 1.38 4.88 1.71 3.24 3.19 4.35 4.35 3.09
Sm 2.99
0.93 1.26 0.43 1.22 1.18 1.37
0.83 1.46
0.13 0.69 1.03
Eu 0.12 0.13
2.54
0.48 0.65 0.63 2.21 1.47 4.57 2.11 3.95 3.85 4.45 4.72 3.41
Gd
0.30 n.d. n.d. 0.64 0.62 0.62
Tb 0.06 0.11 0.08 0.30 0.31 0.72 0.53
2.05 3.78 1.89 3.67 3.65 3.35
1.38 3.91
0.26 0.57 0.20 0.58 1.57 2.14 0.90 1.66 1.67 1.51 1.80 1.54
Yb
0.24 0.28 0.17 0.24 0.24 0.21
Lu 0.04 0.09 0.03 0.10 0.10 0.24 0.22
41.47 124.19* 35.03* 42.01 66.04 107.19
128.23 97.04
16.19 37.97
SREE 18.08 27.75 96.76
29.94
8.51 3.91 19.55 24.57 2.96 7.75 3.09 1.72 3.93 6.54 5.42 1.56
LaN/YbN
3.14 3.18 1.50 0.82 1.92 2.12 2.09 0.72
LaN/SmN 3.62 2.97 4.10- 4.44 5.26
3.16 0.76 1.72 1.91 1.92 1.86 2.38
3.08 2.12
0.92 2.55 1.79
GdN/ 1.49 YbN
0.70 0.59 0.53 0.86 2.00 0.82 0.69 1.04 1.03 0.95 0.99 0.97
Eu/Eu* 0.82
4.41 7.46 4.91 2.97 3.02 3.53 3.17
Th 2.21 5.97 6.30 9.82 6.21 5.22
0.88 1.01 0.92 0.37 0.53 0.59
1.18 0.81
2.69 0.75
U 0.81 1.90 1.52
Th/U 2.7 3.1 4.1 3.7 5.3 5.0 7.4 5.3 8.03 5.7 6.0 3.9 7.0
P
Chondrite-normalized REE contents in komatiite, tonalite, rhyolite and in quartz-rich rocks from Hisovaara and calculated compositions of mixtures that result with various dilutions by quartza
6 7
1 2 3 4 5 8 9 10 11 12 13 14 15 16 17 18 19
La 2.17 49.43 117.9 4.58 4.60 12.51 12.55 8.01 7.97 17.62 17.64 30.78 31.14 23.67 24.66 29.27 28.82 86.30 89.92 10.33 10.04 6.82 6.58 14.89 14.11 24.27 24.92 19.85
3.86 19.52
Ce 2.48 40.80 92.21 4.06 21.19 22.65 68.25 70.63
Pr 2.18 34.47 69.50 3.42 3.28 8/09 7.86 5.39 5.56 11.00 11.06 19.09 19.77 15.15 15.12 17.84 17.29 52.28 53.88 6.20 5.87 4.14 4.24 7.87 8.26 14.08 14.59 11.10
2.61 11.02
2.64 12.83 12.49 38.03 38.93
Nd 2.98 26.32 49.87
3.77 3.13 2.60 2.56 4.42 4.41 7.60 7.62 5.78
Sm 3.77 15.91 22.40 1.75 1.76 5.40 6/04 5.84 19.42 18.19
2.07 1.92 1.38 1.58 2.76 2.70 3.62 4.78 2.24
1.03 3.41
Eu 1.72 9.83 14.31 1.03 2.59 3.71 11.90 11.56
2.06 1.90 1.62 1.48 2.69 2.68 4.45 4.33 3.08 3.04 3.18 3.26 10.82 10.16 Gd 3.87 9.20 12.43 1.08 1.17
0.94 1.11 0.87 1.02 1.34 1.56 2.40 2.49 1.42
0.84 1.58
3.07 1.30 1.40 5.12 4.35
Dy 6.34 5.43 0.71
0.66
2.71 4.70 4.10 0.66 0.84 0.87 0.78 0.76 0.84 1.22 1.99 1.87 1.08 1.18 1.02 1.17 3.49 3.64 Er
0.82
3.57 5.63 3.94 1/03 1.03 0.99 0.91 0.90 1.51 1.39 2.06 2.08 1.21 1.26 1.15 1.17 3.51 3.68 Yb
1.18 1.13 1.18 1.02 1.57 1.59 2.36 2.26 1.18
0.92 1.33
Lu 4.72 6.30 3.94 1.18 1.18 1.22 3.94 3.80
12.15 11.11
La/Yb 0.61 8.82 29.92 4.32 5.61 8.82 8.85 11.64 12.69 14.95 14.99 19.56 19.57 25.43 24.63 24.57 24.43 3.32 4.01 3.09 3.11 4.00 4.00 4.05 4.08 4.10
2.61 4.57
2.61 4.85 4.93 4.44 4.94
La/Sm 0.58 3.11 5.26
2.00 1.68 1.78 1.64 1.78 1.93 2.16 2.08 2.55 2.41 2.76 2.79 3.08
Gd/Yb 1.08 1.64 3.15 0.02 1.43 2.76
0.74 0.79 0.67 0.81 0.80 0.78 0.62 0.83 0.53 0.84 0.59 0.85 0.82
0.72 0.85
Eu/Eu* 0.45 0.81 0.86 0.75
a(1) Komatiite (s.576-4); (2) tonalite (s.831-1); (3) rhyolite (94-PCT-.024); (4) quartz arenite (94-PCT-.011); (5) mixture of komatiite: tonalite
=1; 1, diluted 5.6 times by quartz; (6) quartz arenite (94-PCT-010); (7) mixture of komatiite: tonalite: rhyolite:1:1:1 diluted 4.4 times by quartz; (8) quartz arenite (94-PCT-007); (9) contents in tonalite diluted 6.2 times by quartz; (10) pebbly quartz arenite (94-PCT-003); (11) mixture of komatiite:tonalite:rhyolite:1:1:1 diluted 3.1 times by quartz; (12) pebbly quartz arenite (94-PCT-012); (13) mixture of tonalite:rhyolite
Fig. 15. Chondrite-normalized REE diagrams for tonalite, komatiite, rhyolite, some quartz-rich rock samples and rated mixtures at various degrees of quartz dilution. The normalized values given in Table 3 were used. Sample numbers in this figure are those used in Table 3.
Fairly aggressive acid rains, related to fumarolic activity between volcanic paroxysms, could well be responsible for the development of weathering on andesites.
In summary, the Hisovaara quartz arenites are associated with intermediate to felsic volcanics
3.3. Felsic fragmental rocks(unit F)
3.3.1. Field description
This unit is composed of felsic pyroclastic rocks
and subordinate reworked equivalents
(Kozhevnikov, 1992). In this section we provide the field evidence for interpretation of the deposi-tional environment of the unit. In the vicinity of location B, the sequence consists of metre scale units of tuff breccia and lapilli tuff with occa-sional tuff intercalations. Primary textures and structures indicate the presence of both pumice and lithic fragments. Each unit is defined by distinct size ranges of clasts, texture of pumice, a distinct phenocryst content and a regular succes-sion of size and density grading. Within individual units as defined above, reverse grading of pumice, normal grading of lithic fragments, sporadic ex-amples of basal ground surge beds and thinly
laminated fine ash tuff beds occur which
indicate the units represent ignimbrite deposition (Sparks et al., 1973). Evidence for welding con-sists of the presence of branching fumarolic struc-tures (cf. Thurston, 1980) and the presence of flattened silicified pumice fragments concentrated toward the middle of the stratigraphic unit. The
silicified pumice ignores depositional unit
boundaries and is thus interpreted as evidence of vapour phase recrystallization (Ross and Smith, 1961) indicative of subaerial eruption and deposi-tion.
At location A, in a few exposures, the quartz arenite is overlain by a few metres of a fragmental aluminous metaconglomerate with granitoid and possible metavolcanic clasts. This is overlain by about a 100 m thickness of thin graded beds of sulfidic argillite and carbonate bearing silty sandstones.
3.3.2. Petrography
The rhyolites at location B consist of varying proportions of quartz, plagioclase and minor potassium feldspar with accessory biotite and sericite. Primary textures are completely obliter-ated by metamorphic recrystallization but gross grain size variation seen mesoscopically is present in thin section. Silicified pumice fragments contain irregular polycrystalline plates of quartz whereas
the matrix contains finer rained quartz, feldspar, biotite and sericite.
3.3.3. Geochemistry
The sole analysis of the rhyolitic unit done for the present study shows the unit to be a high silica rhyolite with potassium dominant over sodium (Table 3). In trace element terms, the unit displays a fractionated REE pattern similar to the FI rhyolites of Lesher et al. (1986).
4. Summary and conclusions
The Hisovaara quartz arenites represent a mixed provenance involving contributions from TTG suite granitoids and a mafic to ultramafic component with extensive weathering to explain the lack of feldspar in the sandstones. Mature, quartzose, shallow-water sandstones are not com-mon in Archean greenstones (Thurston and Chivers, 1990; Lowe, 1994). The quartz-rich sand-stones at Hisovaara are unusual in showing clearly the base of the sequence and evidence for weathering of the andesitic basement seen in field, petrographic and geochemical evidence. Most Archean quartz-rich sandstones are associated with platforms (De Kemp, 1987; Thurston and Chivers, 1990) with one example of a cannibalized platform within a submarine fan environment (Cortis, 1991). As an Archean quartz rich sand-stone sequence, the Hisovaara quartz arenites are closely associated with subaerial arc andesites and subaerial rhyolites at the south end of the belt similar to the ‘continental’ style assemblage type of Thurston (1994). However, at the north end of the belt, the quartz arenites are succeeded upward by conglomeratic rocks, argillites, and an overly-ing tholeiite unit with komatiites. This end of the unit is then comparable to some of the Superior
Province platformal quartz arenites in that
Post Archean quartz-rich sandstones are
con-ventionally considered to represent multiple
passes through the sedimentary cycle (Pettijohn et al., 1972, p. 298) with the interplay of climate, relief, and provenance influencing the composi-tion of the sands (Basu, 1985). Recent work in the Orinoco basin has demonstrated the production of single cycle quartz arenites in a regime of intense chemical weathering (Johnsson et al., 1988). The process involves either long soil resi-dence times related to very low erosion and trans-port rates or storage of orogenically derived sediments on alluvial plains enroute to the final depositional site. In spite of the variety of mecha-nisms for production of quartz rich sandstones, we here use their presence in the Hisovaara green-stone belt to indirectly indicate the presence of granitoid rocks in the source area. A granitoid source area serves as a speculative indicator of at least unroofing of plutons if not a possible cra-tonizing or orogenic event. If the latter is the case, the age constraints available in this greenstone belt suggest the possibility of a pre-2.7 Ga oro-genic event in the Baltic shield. Much additional work is required to validate such a concept, but the tantalizing indication seen in this project will perhaps point the way to further work on this speculation.
5. Geochemical methods
Sample preparation and major element analysis by X-ray fluoresence were carried out at the Kare-lian Research Centre. The XRF analyses were determined on fused glass discs after the method of Norrish and Hutton (1969). Trace elements were determined in the Geoscience laboratories of the OGS at Sudbury. Rb, Sr, Ba, Cr, Ni, Zr and Y were determined by XRF on pressed powder pellets. All other trace elements were determined by ICP-MS following a mixed acid digestion in open beakers (OGS, 1990). Results were checked for dissolution problems by comparison of XRF determined Zr vs. ICP-MS values. Precision on reference materials has been at the level of
B10% using these methods (cf. Tomlinson et al.,
1998).
Acknowledgements
This project is an outgrowth of the US – Rus-sia – Canada co-operative program during which the senior author visited Hisovaara with OGS support. At that time the andesite-quartz arenite assemblage was noted in the field as unusual. Subsequent field work was funded by the Geolog-ical Institute of the Karelian Research centre, Karelian Branch of the Russian Academy of Sci-ence in 1994 and 1996. We thank Dr Sergei I. Rybakov, Director of the Geological Institute for his support of the project. This paper is published with the permission of the Senior Manager Pre-cambrian Geoscience Section Ontario Geological Survey (Ontario Geological Survey, 1990). This project would not have been possible without translation on the outcrop by Grigori N. Sokolov of the Institute and subsequent translation of e-mails, letters and drafts of the paper. Dr K.I. Heiskanen of the Institute is gratefully acknowl-edged for his thoughtful review of an early ver-sion of the manuscript. R.W. Ojakangas and J. Dostal as journal reviewers helped clarify many points and sharpen the presentation. Major ele-ment analyses were done in the chemical labora-tory of the Institute of Geology of the Karelian Research Centre (Saraphanova, R. Ph., Mokeeva, L.N., Punka G.P., and Pitka, N.V.) Field assis-tance was provided by E. Travina in 1994. Draft-ing has been done by O. Kozhenikova and S. Josey.
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